Purpose of review: Energy expenditure is measured under a number of conditions. These include resting metabolic rate (a minimum level of metabolism), maximal metabolic rate (attained during maximal exercise), as well as sustained (and endurance) metabolism, when extended periods of activity are fuelled by either food intake or energy reserves. The factors that determine each type of metabolic rate are not the same; however, all levels of energy expenditure are strongly linked with body size.
Recent findings: Resting metabolism in different species is related to the fatty acid composition and physical properties of membranes and consequently their influence on the molecular activity of membrane proteins. Maximal metabolism (during aerobic exercise) is primarily limited by oxygen transport and delivery to the working muscles in most animals. In the most metabolically active animals it may be related to design limits of muscles and transport systems. Sustained and endurance metabolism are largely constrained by factors related to supply, storage and utilization of energy substrates.
Summary: Animals display great diversity in physical and metabolic performance. The many factors that interact to set the upper limit of performance in different species are set by their evolutionary history and define the metabolic window in which they exist.